Lyme disease, caused by Borrelia burgdorferi, is the most prevalent arthropod-borne disease of humans in the United States and many other countries throughout Europe and Asia. Tick-borne relapsing fever, caused by Borrelia hermsii, is endemic in scattered foci throughout many regions of higher elevation in the western United States Our work to improve on the serodiagnosis of both Lyme disease and tick-borne relapsing fever has used recombinant DNA technology to clone genes of spirochetes that express proteins that induce specific and detectable antibody responses in humans infected with borreliae Our studies examining how spirochetes adapt to their tick and mammalian hosts require that we maintain colonies of both Ixodes scapularis and Ornithodoros hermsi, the respective tick vectors of Lyme disease and relapsing fever spirochetes, and infect these ticks via a laboratory mouse - tick cycle. Both species of ticks are infected with spirochetes, and then later examined by immunofluorescence staining for the distribution of spirochetes within ticks. Antisera and monoclonal antibodies specific to various outer surface proteins allow us to use fluorescence microscopy to determine how the surface of these bacteria change during tick feeding and as the bacteria are transmitted from tick to mammal and from mammal to tick. Our collection of new, low passaged isolates of Borrelia hermsii have allowed us to determine DNA sequence variation of potential diagnostic and protective antigens compared to highly conserved genes that reflect proper phylongenetic relationships. Four genes have been sequenced from 25 isolates of Borrelia hermsii, which demonstrate the existence of two distinct taxonomic groups with considerable heterogeneity in the target antigens of interest.